In the United Sates of America and the likes in which a large amount of electricity is consumed, the generation of electric power using the so-called simple cycle gas turbines, for which gas turbines are installed and operated independently, has been increased in order to supplement electricity and to cope with a time when the amount of electricity used becomes a peak. Since the equipments used for the electric power generation are constructed in suburbs of towns and cities, it is necessary to decompose NOx contained in exhaust gases at a high ratio to purify the gases. On the other hand, in the electric power generation using simple cycle gas turbines, it is necessary to install a denitrating apparatus just behind the exit of each gas turbine to treat an exhaust gas at a high temperature of 450° C. to 600° C. However, denitration catalysts having high performances and a long life at such a high temperature have not been obtained heretofore. Since the lowering of the activity of catalyst due to its deterioration by heat is remarkable especially at a high temperature region, it was necessary to treat an exhaust gas by using a catalyst having a low activity but an increased thermal resistance at the sacrifice of activity and packing the catalyst in an amount several times as large as that used for denitrating an exhaust gas discharged from a boiler and having a temperature of about 350° C. Accordingly, even in an equipment constructed to cope with a time when the amount of electricity used becomes a peak and having a short annual operation time, it is necessary to provide a large denitrating apparatus. Thus, it has come to a heavy burden on society even economically.
Accordingly, many inventions of catalysts which are hardly deteriorated at high temperatures have been made, and it has been known particularly that catalysts having cerium (Ce) as an active component exhibit comparatively high performances even at high temperatures. For instance, a catalyst which is prepared by coprecipitating a soluble titanium (Ti) compound, tungsten (W) compound, and cerium (Ce) compound so that the particles of cerium compound are highly dispersed in titania is disclosed in Laid-open Japanese Patent Publication No. Hei 08-257402. Further, for intending to increase the activity and stability of a catalyst prepared by dispersing Ce ions within micro voids of a zeolite by ion exchange method is disclosed in Laid-open Japanese Patent Publication No. Hei 08-27408. Whereas these catalysts have an excellent side from the purpose of increasing the stability and activity of catalyst by highly dispersing Ce, which is known as an active component of a catalyst for a long time, in the catalysts, they have problems remained to improve from the purpose of providing a catalyst having a high activity and an increased heat resistance at a high temperature region, pursued by the present inventors.
Among the conventional technologies described above, a method for preparing a catalyst by precipitating soluble compounds of Ti, W, and Ce by coprecipitation have problems as follows:    i) As a premise, Ti and Ce have the same valence of 4 and the compounds comprising one of both the elements, respectively, are ready to form a state wherein the compounds are uniformly dispersed in each other when they are mixed. Thus, when mixed, the compounds form a state wherein a large portion of a Ce compound (oxide) is embedded in a Ti compound (oxide) so that it is impossible to sufficiently exploit the excellent activity of the Ce compound. This conclusion can be inferred from the fact that the separation of both the compounds from each other is difficult and a Ce oxide remains in an industrial titanium oxide frequently in an amount of nearly 1% as impurity.    ii) A precipitate obtained by coprecipitation method presents a gel-like state and can hardly be subjected to filtration. Thus, it is necessary to pass the gel-like precipitate through many complicated steps by the time when the precipitate can actually be used as solid catalyst. Accordingly, the coprecipitation method has such a drawback that the production cost of a catalyst becomes high.
Besides, in a method in which Ce ions are highly dispersed within micro voids of a zeolite, whereas a catalyst having an extremely high initial performance can be obtained, the catalyst is easily deteriorated by the so-called de-aluminum phenomenon wherein the aluminum contained in a zeolite is separated from the zeolite structure into the micro voids of the zeolite. Since the de-aluminum phenomenon is accelerated especially by a high ambient temperature and the presence of steam, when the catalyst is used for a denitration at a high temperature wherein the catalyst is exposed to an exhaust gas containing 5 to 10% of steam for many hours, it is difficult to maintain a high activity of the catalyst for a long period of time.
In consideration of the problems existing in the prior arts, the subject matters of the present invention is to provide a denitrating catalyst in which a Ce oxide is prevented from being embedded in a Ti oxide to realize such a high degree of dispersion of the Ce oxide on the surface of the Ti oxide as comparable with the case wherein cerium ions are dispersed within micro voids of a zeolite by ion exchange method, and which catalyst is free from occurring such phenomena as sintering of the Ti oxide and deterioration of the zeolite with steam. For instance, the present invention is to provide a method by which NOx contained in a high temperature exhaust gas exhausted from such an equipment as that for electric power generation using simple cycle gas turbines is treated to render into harmless.